For thin steel sheets for use in framework components of cars, higher strength is required for attaining car crush safety and improvement of fuel cost. For this purpose, while it is demanded to increase the strength of a steel sheet to 980 MPa grade or more, since forming load increases during pressing, this involves a problem that an excessive load is applied to a press. Accordingly, it has been demanded for the development of a steel sheet having a low strength during forming and a high strength during use after the forming. Then, as means for reducing load during forming, warm forming has been known (for example, refer to patent literatures (PTL 1 and 2).
PTL 1 discloses a high-strength steel sheet for use in warm forming having a ratio of a tensile strength at 450° C. to a tensile strength at a room temperature of 0.7 or less. However, the high-strength steel sheet for use in warm forming less lowers a tensile strength at 150° C. (refer to paragraph [0056], Table 3) and has to be formed in a relatively high temperature region of from 350° C. to an A1 point in order to obtain a sufficient effect of reducing the load during forming (refer to paragraph [0018]). Accordingly, the steel sheet involves a problem that the surface state of the steel sheet is impaired due to oxidation and consumption of energy for heating the steel sheet is increased. Further, it may be possible that the strength during use after the forming may be lowered due to annealing of martensite.
PTL 2 discloses a cold rolled steel sheet containing, on the basis of mass %, 0.040 to 0.20% of C, 1.5% or less of Si, 0.50 to 3.0% of Mn, 0.10% or less of P, 0.01% or less of S, 0.01 to 0.5% of Al, 0.005% or less of N, and 0.10 to 1.0% of V with the balance consisting of Fe and unavoidable impurities and which is further suitable to warm forming where 90% or more of V is in a solid solution state. However, the cold rolled sheet has to be worked in a relatively high temperature region of 300° C. or higher and an A1 point or lower (refer to paragraph [0021]). Accordingly, the steel sheet involves a problem that the surface state of the steel sheet is impaired due to oxidation and consumption of energy for heating the steel sheet is increased in the same manner as the high tensile strength steel sheet for use in warm forming described in PTL 1. Further, since expensive V has to be added, it also involves a problem of increasing the cost.